1. Field of the Invention
The invention relates to a method for saving register space, particularly to a method for saving register space in a conventional high-level function call process, which increases the utility of registers when a function call is coded by a high-level compiler, thus increasing the function call efficiency of a software program.
2. Description of Related Art
In a conventional high-level function call, parameters are methodically stored in specific registers in order to pass them to a callee, or methodically stored in a memory stack in order to pass them to the callee. To increase the cited performance, a high-level compiler commonly stores all or partial parameters in registers for passing to the callee, thus speeding call process.
Existing processors generally have multiple 32-bit registers. However, data types for operations are different in high-level languages. For example, data types exist comprising 8, 16 and 32 bits. One manner for increasing utility is to split a single register to several parts like an upper and lower word in some registers. For example, registers R0-R7 in a Register File respectively have 32 bits, in which R0L representing lower 16 bits and R0H representing upper 16 bits can be operated separately. Thus, a high-level compiler generally selects R0 for 32-bit data and R0L or R0H for 16-bit data. However, if the 32-bit data is stored in two registers such as R0H and R1L, the compiler cannot access both at a time and proceeds further in operation.
FIG. 1 is a schematic diagram of a conventional parameter placement for pass. As shown in FIG. 1, a conventional high-level compiler methodically places parameters Arg1, Arg2, and Arg3 in registers when compiling program codes for a function call so that a waste of register space occurs. For example, data types for parameters Arg1, Arg2 and Arg3 respectively are 16, 32 and 16 bits. The 16-, 32- and 16-bit data are methodically stored in first, second and third 32-bit registers R0, R1 and R2. As such, a total of 32 bits (R0H and R2H) are wasted. However, this problem will continue because the high-level compiler has to obey the predetermined rules to compile function calls to ensure exact coding between a callee and callee program.
Accordingly, an object of the invention is to provide a method for saving register space in a conventional high-level function call process, which increases the utility of registers when a function call is coded by a high-level compiler, thus increasing the function call efficiency of a software program.
The invention provides a method for saving register space in a conventional high-level function call process, which can place parameters from either of two ends of a sequence of registers for parameter pass, based on data length, thereby reducing register waste. The method includes methodically arranging a sequence of registers for parameter pass, sequentially naming every k bits in the sequence from left to right as a first group, sequentially naming every k/2 bits in the sequence from right to left as a second group, and sequentially placing input parameters with a width exceeding k/2 in the first group from left to right or conversely with a width less than or equal to k/2 in the second group from right to left.